FIG. 9-FIG. 11 show conventional motors. FIG. 9 shows a sectional view of a conventional round motor, FIG. 10 shows a sectional view of a conventional flat motor, and FIG. 11 shows another conventional flat motor.
The round motor shown in FIG. 9 is a brushless motor, and formed of stator 101 and rotor 102. Each one of a plurality of teeth 103 of stator 101 is wound with a winding in a concentrated manner. Stator 101 has six slots. Rotor 102 includes permanent magnets 111 in which eight poles are available. The conventional motor shown in FIG. 9 thus forms a permanent-magnet motor having 8 poles and 6 slots.
A stator core of stator 101 comprises teeth 103 arranged radially, and arc-like yoke 104 coupling respective teeth 103 to each other. Each one of teeth 103 has tooth-end wider section 105, having a wider width along the rotational direction, at its inner end. Between the adjacent wider sections 105, slot open 106 is formed, and between adjacent teeth 103, slot 107 is formed. Each one of teeth 103 is wound with winding 109 via insulator 108. Windings 109 regularly employ three-phase windings.
Rotor 102 is rotatably held inside stator 101 via clearance in between, and concentric with stator 101. Rotor core 110 is mounted with ring-shaped permanent magnets 111. A rotary shaft is inserted into shaft hole 112. Rotor 102 is rotated on the rotary shaft with a rotating magnetic field generated by an electric current running through windings 109 provided to stator 101.
The flat brushless motor shown in FIG. 10 has been designed for downsizing the round brushless motor shown in FIG. 9 and yet for obtaining higher power. A structure of this kind of flat brushless motor is disclosed in, e.g. Unexamined Japanese Patent Publication No. 2002-136090. Flat brushless motor 120 shown in FIG. 10 includes teeth 123 arranged in parallel with each other; however, teeth 123 have different shapes, so that cogging torque tends to occur.
FIG. 11 shows another conventional flat motor, which produces less cogging torque than the motor shown in FIG. 10. In FIG. 11, elements having the same functions as those shown in FIG. 9 have the same reference marks, and the descriptions thereof are omitted here.
Stator 113 of the motor shown in FIG. 11 includes a plurality of teeth 103 radially arranged, arc-like yokes 104 and linear yokes 114 coupling respective teeth 103 to each other at the outer walls of teeth 103. Two arc-like yokes 104 and two linear yokes 114 are alternately connected to each other so that two linear yokes 114 can be in parallel with each other. This structure forms the motor flat, which allows providing a compact brushless motor outputting higher power.
However, the foregoing shape of stator 113 sets a limit to a sectional area of the windings at flat sections because the sectional area of each one of three-phase windings differs from each other, and the entire windings are thus determined under this limitation. As a result, the higher power of the flat motor shown in FIG. 11 is limited, so that it may happen that necessary power cannot be obtained from the motor.